1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus including the sheet conveying device.
2. Description of the Related Art
Some image forming apparatuses employ intermediate transfer system in which a primary transfer unit primarily transfers a toner image from each of photosensitive elements onto an intermediate transfer medium, thereby forming a full-color image on the intermediate transfer medium, and a secondary transfer unit secondarily transfers the full-color image onto a sheet-like recording medium (hereinafter, “sheet”). The image forming apparatuses can be, for example, printers, facsimiles, and copiers; and the intermediate transfer medium can be, for example, a drum and a belt. The intermediate-transfer image forming apparatuses have an advantage in wide availability of types of the sheet including thin paper, thick paper, postcards, and envelopes.
However, when a sheet with a predetermined value or larger thick (hereinafter, “thick sheet”) is fed into the secondary transfer unit, a rotating speed of the intermediate transfer medium, which keeps constant until just before the thick sheet is fed, fluctuates in a short time. This fluctuation results in a distortion of the image by the primary transfer unit.
Some intermediate-transfer image forming apparatus includes a transfer fixing unit that transfers the toner image from the intermediate transfer medium onto the sheet, and, at the same time, fixes the toner image onto the sheet. Even in such image forming apparatuses, when the thick sheet is fed into the transfer fixing unit, the rotating speed of the intermediate transfer medium, which keeps constant until just before the thick sheet is fed, fluctuates in a short time. This fluctuation results in a distortion of the image by the primary transfer unit or the secondary transfer unit.
In a field of image forming apparatuses that transfers the toner image from the photosensitive elements or the intermediate transfer medium onto the sheet, with the downsizing of the image forming apparatuses, a transfer unit tends to be arranged closely to a fixing unit. In some image forming apparatuses, especially, the toner image is transferred onto the sheet, and, at the same time, fixed onto the sheet. Even in such image forming apparatuses, when the thick sheet is fed into the fixing unit, the rotating speed of a fixing roller or a fixing belt, which keeps constant until just before the thick sheet is fed, fluctuates in a short time. This fluctuation causes non-smooth conveying of the sheet, which results in a distortion of the image by the transfer unit.
Japanese Patent Application Laid-open No. 2005-107118 discloses a technology for suppressing the fluctuation in the rotating speed of the intermediate transfer medium occurring when the sheet is fed into the secondary transfer unit, i.e., a nip between the intermediate transfer medium and a secondary transfer roller. More particularly, the speed compensation is started at predetermined proper timing so that the rotating speed of the intermediate transfer medium is increased from a standard value by a predetermined offset value. Because an amount of the fluctuation depends on thickness of the sheet, the speed compensation is performed in consideration of the thickness of the sheet that is fed into the secondary transfer unit. Thus, occurrence of the fluctuation in the rotating speed is suppressed properly in consideration of the thickness of the sheet. As a result, the rotating speed of the intermediate transfer medium keeps constant.
However, inventors of the present application found that, even if the thickness of the sheet is fixed, the amount of the fluctuation in the rotating speed occurring when the sheet is fed into the secondary transfer unit is variable depending on width of the sheet. Assume, for example, that an A4-size sheet is fed into the secondary transfer unit or the like in the portrait orientation in a first case and the same sheet is fed in the landscape orientation in a second case. The amount of the fluctuation in the rotating speed in the first case is different from the amount of the fluctuation in the second case. Therefore, to compensate the fluctuation in the rotating speed of the intermediate transfer medium or the like properly, it is necessary to use the offset value that is adjusted in consideration of the sheet width.
Moreover, even if the sheet thickness and the sheet width are fixed, the amount of the fluctuation in the rotating speed of the intermediate transfer medium or the like occurring when the sheet is fed into the secondary transfer unit or the like is variable depending on a sheet conveying speed. Therefore, if the image forming apparatus can selectively change the sheet conveying speed, it is necessary to use the offset value that is adjusted in consideration of the sheet conveying speed to properly compensate the fluctuation in the rotating speed of the intermediate transfer medium or the like.
Still moreover, if the sheet width, the sheet thickness, and the sheet conveying speed are not fixed, it is necessary to use the offset value that is adjusted in consideration of all the sheet width, the sheet thickness, and the sheet conveying speed to properly compensate the fluctuation in the rotating speed of the intermediate transfer medium or the like.